A cross-correlation operation for purposes of recognition might consist of the following: An image of interest is focused onto a reference pattern, perhaps a positive photographic transparency. If the image and pattern contain some region which is the same, then they can be rotated and translated with respect to one another until this region is in coincidence for both image and pattern. Generally, the amount of image light passing through the reference pattern is greater for coincidence than non-coincidence. The peak in light transmitted as a function of image-pattern displacement is called the correlation peak. The image light transmitted through the pattern outside the region of coincidence is generally non-zero and fluctuates as a function of displacement; this light contribution will be referred to as optical noise. For recognition purposes it is desirable to reduce this noise as much as possible. Noise is relatively small compared to the correlation peak when the image (pattern) is complex and the total area of bright (transmitting) features is small compared to that of the dark (opaque) regions. An array of bright stars against darkfield space satisfies this condition, but daytime terrain, etc. generally does not. However, the terrain image (pattern) would be appropriate if each feature were replaced by a thin, bright (transmitting) outline and if all solid areas, dark or light, were made black (opaque). It is exactly this change that will be referred to as a transformation to an edge-enhanced, darkfield representation.
Cross-correlation operations are applicable to recognition and identification in character-reading machines, aerial reconnaissance, image-recognition missile guidance, etc. Edge-enhanced, darkfield representations can be obtained by using coherent light, very high quality optics, and spatial filtering techniques. Cross-correlation operations using coherent light can be performed by comparing an appropriately filtered input image with either a reference pattern or a Fourier transform filter made from it which displays spatial frequency content. Where real-time processing is required, the incoherent-light input image must be transferred to a coherent beam by using a suitable real-time device, e.g., a liquid crystal optical modulator which is under development by Hughes.
The disadvantages of an expensive, fragile, and complex coherent optics system in performing cross-correlation operations, particularly with small image-guided missiles, is evident. One alternative to coherent optics image-guidance of a missile is the laser target designator. A laser beam illuminates the target, and the missile, sensing this light, is guided to it. The disadvantages of this approach is that a laser transmitter (a location give-away) is required, and the target must remain illuminated during the missile's entire flight.
A concept based on recognition of targets, characters, terrain, etc., using reference patterns, electronics and incoherent light from the object would certainly have many potentially economical applications. The purpose of this disclosure is to describe systems which are pertinent to such a concept. System Type I obtains an edge-enhanced, darkfield representation of an incoherent-light input image, and System Type II produces a cross-correlation signal for an edge-enhanced, darkfield input image with an edge-enhanced, darkfield reference pattern without the need to actually display the transformed input image.
For applications in aerial reconnaissance and missile guidance where edge-enhanced, darkfield representations of terrain/target configurations are required, this invention can serve to reduce cost and fragility (eliminating on-board coherent optics), reduce likelihood of being discovered (no transmitter used) and, appropriately used in image-recognition missile guidance systems, promote a "fire and forget" capability not posessed by systems depending on continuous laser target designation during missile flight. Applicability to character reading machines, and a host of other optical processing systems is evident.